One special feature offered in mobile communication systems is group communication. The term “group”, as used herein, refers to any logical group of two or more users intended to participate in the same group communication. One example of group communication is a group call, which is a call in which all participants may take turns to speak and to listen to each other.
Conventionally, group communication has been available only in trunked mobile communication systems, such as Professional Mobile Radio or Private Mobile Radio (PMR) systems, such as TETRA (Terrestrial Trunked Radio), which are special radio systems primarily intended for professional and governmental users. Thanks to the evolvement of communication technology, particularly IP-based communication technology, and end user equipment, a group communication service is now available also in public mobile communication systems. Examples of services providing group communication service include Push-to-talk over Cellular (PoC), messaging (IM), and conferencing, all implemented using primitives provided by SIP (Session Initiation Protocol) which is not vertically integrated into a communication system but a tool to build a multimedia architecture. Such group communication may include data calls, audio calls, video calls, multimedia calls, messaging, electronic mail, etc.
Due to the versatile communication possibilities, different media types can be usable in a group, such as a Voice PoC call, SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) IM chat, Video PoC call and Gaming Service. However, for the time being no mechanism exist with which group members can be informed about usable media type(s) in a group. (A group member is a user intended to participate in the group communication.) Thus, group members have to guess the usable media type(s) in the group.